Radio frequency (RF) power amplifiers are utilized to increase or amplify the radio frequency energy applied to a radio antenna for subsequent radiation. Because such amplifiers are relatively high power circuits, issues such as thermal efficiency and related matters must be carefully considered by practitioners. Some radio systems, such as those employing frequency or phase modulation wherein information is carried by modulating the frequency of a radio carrier rather than the amplitude, can advantageously use a radio frequency power amplifier operating in a class C (switched) mode.
Such a radio frequency power amplifier is typically more efficient than an amplifier operating in a class A mode (the amplifier is conducting a non zero bias current, or is direct current (DC) biased) and hence will consume less energy from, for example, a battery source. Theoretically a class C mode amplifier can be modeled as a switch that is toggled between an "on" and an "off" state at the radio signal's frequency. By selectively filtering the input and output wave forms to and from this switch the desired frequency signal may be obtained and the harmonics of the toggling frequency may be rejected. The duration of the "on" state will determine the output power from such an amplifier when other parameters are fixed. The advantage of the class C amplifier lies in the theoretical efficiency that may be obtained. Essentially when the switch is "off" no current flows and when the switch is "on" the potential across the switch is zero. Since power dissipated is the product of current times potential, the switch will theoretically dissipate zero power and the class C amplifier will be very efficient.
In practice the switch in a class C amplifier is a semiconductor transistor that has been configured as a power amplifier device. This switch unlike the model is not ideal. For example, the potential across the device will be nonzero because the transistor will have some forward resistance. Additionally, the transistor will have a current transfer characteristic that is often modeled as an exponential or other power law relationship between drive (base) current and load (collector) current at least during a nonzero switching time period. Furthermore most semiconductor devices exhibit a turn-on threshold that necessitates a minimum amount of bias or alternatively signal drive level before the device will provide any useful switching or amplification action. These issues among others, contribute to some problems for class C radio frequency power amplifiers, particularly during the period of time when a transmitter is being turned on.
During this time period when the amplifier is ramping up from zero output power to something approaching a final output power level the amplifier may generate spurious signals, such as harmonics or various combinations of harmonics of the drive signal frequency. These spurious signals may have energy components in spectral locations where other transmitters and receivers are authorized to operate. This spurious energy can cause radio interference that degrades the overall effectiveness of radio communications in these spectral locations and this may result in a violation of the regulations of various radio regulatory agencies, such as the Federal Communications Commission, and hence possible sanctions for the operator of the offending radio. Historically this has not been a major problem because the period of time when interference might exist was comparatively short relative to the total time the transmitter was turned on particularly given the relative infrequency of turning the transmitter on.
However, given the increased demands being placed on available spectrum, practitioners are proposing systems such as packet data systems and Time Division Multiple Access (TDMA) systems where the transmitter is turned on for a short period of time relatively often. In addition the spectrum where the spurious energy may fall is more and more likely to be occupied and regulated. Practitioners have proposed various methods of providing some degree of control over the characteristics of the amplifier during its ramp up period.
One known approach includes a power output control loop that gradually increases the allowed power out during a short time period at initial turn on. This approach suffers from the threshold effect, noted above, of semiconductor devices operating in a switched mode. In essence the control loop can not serve its purpose until an output is available from the amplifier. Another approach attempts to bias the semiconductor device in a class A or quasi class A mode for a short period prior to and during a portion of the ramp up period thus avoiding the threshold effect and providing a degree of control over the ramp up process. This approach results in some unnecessary battery drain and further because of the architecture and thus characteristics of semiconductor devices optimized for a switching function may contribute to reliability problems with the device.
Clearly a need exists for a radio frequency power amplifier that minimizes the spurious energy emitted during the time period when the amplifier is ramping up.